A standard catalytic converter, especially for motor vehicles, comprises a metal housing with a catalytic converter element positioned on the inside. A ceramic catalytic converter element, in the following called a monolith, has a far lower stability than a metallic one. In addition, the heat expansion coefficients of the ceramic material and the metallic housing are very different. The monolith is therefore positioned inside the housing with the aid of a positioning mat, which is inserted with pre-stressing into a gap between monolith and housing. So-called expanding mats are frequently used as positioning mats. These are mineral-fiber mats with exfoliated mica particles embedded therein. Exfoliated mica irreversibly hydrolyzes water vapor at increased temperatures, thereby causing the particles to change to an expanded state. In the expanded state of the exfoliated mica particles, the mat exerts higher restoring forces in radial direction onto the inside surface of the housing and the peripheral surface of the monolith, which is linked to an increase in the ejection force. The ejection force is understood to be the force with which the monolith must be admitted in axial direction to remove it from its positioning or to displace it in axial direction. For understandable reasons, the ejection force should be as high as possible to ensure a reliable positioning of the monolith during the vehicle operation.
Positioning mats that do not contain exfoliated mica are used in addition to expanding mats. Mats of this type essentially consist only of mineral fibers. The radial restoring forces of both mat types are generated in that the thickness of the mat in the uninstalled condition exceeds the gap measure for the gap space between monolith and housing. Whereas the gap enlargement for expanding mats during the operating temperatures of the catalytic converter is balanced out by the expansion of the exfoliated mica particles, the radial pre-stressing of the positioning mat of mineral fibers without exfoliated mica must be high enough to ensure that the monolith is positioned securely, even in the expanded state of the gap. As a rule, the intent is to have the smallest possible gap measure for the gap space in order to increase the restoring forces of a mat with a specified thickness. With housings consisting of two shell halves, a monolith packet consisting of one or several monoliths wrapped with a positioning mat, is initially inserted into one half shell and the second half shell is then fitted on. In the process, the positioning mat must be compressed to the thickness corresponding to the desired gap measure. While a monolith is relatively insensitive to a radially effective isostatic load, there is danger that the monolith is destroyed during the shearing stress that may result from a tangential force introduction. With a housing consisting of two shell halves, a shearing stress of this type occurs mainly along the edges of the half shells. Thus, relatively narrow limits are set for reducing the gap measure of a catalytic converter of this type. The same is true for catalytic converters having a wrap-around housing. A third type of catalytic converter design uses a tube section for the part of the housing where the monolith or monoliths are located. For the production of such catalytic converters, the above-mentioned monolith packet is pressed into a tube section. The restoring forces generated by the compression of the positioning mat in the process uniformly act upon the peripheral region of the monolith, meaning they have a quasi isostatic effect on the monolith. A shearing stress virtually does not occur. Nevertheless, the gap space in traditional tube-shaped catalytic converters cannot be reduced to a satisfactory degree for increasing the mat restoring forces. The reason for this is that pressing the monolith packet into a tube section becomes proportionally more difficult the smaller the available gap space or the greater the positioning mat thickness that exceeds the available gap measure for the gap space.
Starting with this, it is the object of the invention to suggest a catalytic converter with improved positioning of the monolith, as well as a method for producing a catalytic converter with a tubular design.
This object is solved with a method and catalytic converter according to the present invention. If reference is made to an approximately cylindrical tube section or an approximately cylindrical monolith, this also includes oval or polygonal tube sections and monoliths. In addition, a catalytic converter in general is understood to mean a device for cleaning exhaust gases, which can additionally or instead of a monolith include a particle filter or a soot filter. For a method according to the invention, a tube section with several different cross-sectional surfaces on the inside is provided, wherein a monolith packet is pressed in from a tube end with a larger or the largest inside cross-sectional surface or clear width. For example, a tube section can be selected, which has a first longitudinal section with larger inside cross-sectional surface and an adjoining second longitudinal section with smaller inside cross-sectional surface. The larger inside cross-sectional surface is selected such that the insertion of the monolith packet will not present any problems. However, the positioning mat is still compressed to generate the restoring forces. The subsequent longitudinal section with smaller inside cross-sectional surface, on the other hand, is selected so that the highest possible compression of the positioning mat occurs, thus generating the highest possible restoring forces. In contrast, the use of a tube section with on the whole reduced inside cross-sectional surface would engender the danger of the positioning mat getting snagged at the beginning of the pressing-in action, e.g. in the frontal region of the tube section, and that only the monolith would be pushed farther into the tube section. However, if a larger inside cross-sectional surface and accordingly a gap with larger gap measure exist at the pressing-in end of the tube section, the monolith packet can be pressed into the tube section without a change in the desired position of the positioning mat, relative to the monolith. If the front end of the monolith packet that points in pressing-in direction later enters the longitudinal tube section with reduced cross section, the region in front of the positioning mat is already stabilized sufficiently by the tube section, so that a change in the desired mat position is prevented. A tube section pre-manufactured in this way is arranged such that the narrowed longitudinal section encloses the frontal region of the monolith that points toward the inflow funnel.
The production of a catalytic converter according to the invention can also occur such that a monolith packet is pressed from each tube end into the tube section. In that case, both tube ends have a larger cross sectional inside surface than at least one region, arranged in-between, with reduced cross sectional inside surface. It is preferable if a tube section is used, for which the inside cross-sectional surface is reduced in stages, in the form of several longitudinal sections, wherein the inside surface of the respective longitudinal sections extends parallel to the central longitudinal axis of the tube section. In other words, the inside surface of the respective longitudinal section forms a cylinder jacket with circular, oval or polygonal periphery, which extends coaxial to the central longitudinal axis of the tube section. For one embodiment variant, the successive longitudinal sections in pressing-in direction are arranged in the order of decreasing inside cross-sectional surfaces. The positioning mat is compressed more and more with increasing depth for pressing in, until it experiences the highest compression at the end of the pressing-in operation, in the region of the tube end pointing in pressing-in direction.
As alternative to a tube section that is reduced in stages, it is also possible to use a tube section with continuously decreasing or conically tapered inside cross-sectional surface for a longitudinal section. A longitudinal section of this type can extend over the complete length of the tube section. The inside cross-sectional surface continuously decreases from one tube end to the other tube end. The inside surface of a cone-shaped longitudinal section thus forms the jacket surface of a truncated cone, wherein the periphery of this longitudinal section can also be circular, oval or polygonal. The reduction in stages as well as the continuous, cone-shaped narrowing is linked to a stiffening of the tube section or the catalytic converter housing. As compared to the continuous narrowing of the inside cross-sectional surface, a tube section that is reduced in stages has the advantage of resulting in higher friction between positioning mat and tube section.
A tube section 2 comprises longitudinal sections that are conically tapered from its tube ends toward the center. With a tube section of this type, it is useful if respectively one monolith packet is pressed in from each tube end. Finally, it can be advantageous if a tube section comprises at least one longitudinal section with an inside surface that extends parallel to its central longitudinal axis and at least one cone-shaped longitudinal section.
It is furthermore advantageous if the narrowed or the conically tapered longitudinal sections extend only over a partial peripheral region of the tube section. Monoliths with an oval or elliptical cross section can tolerate a higher pressure load in the flat areas, that is to say in the region of the smaller elliptical axis, than in the side regions with higher curvature of the longer elliptical axis. It is therefore advantageous if the total force of pressure is distributed such that the flat sides of the monoliths are subjected to a higher load than the side regions with stronger curvature. To ensure this, a tube section is used that is not narrowed over its total periphery, but only in its regions assigned to the respective flat sides of the monolith. Thus, it is possible to admit the monolith on the whole with an increased radial force of pressure without the danger of a monolith break. The reduction in the above-mentioned peripheral regions can be selected such that following the pressing in of a monolith packet, a uniform gap measure over the complete area is achieved for the gap space.
A variation of the radial force of pressure can be achieved in general in that the narrowing or tapering of the longitudinal sections is more pronounced in one partial peripheral region than in another partial peripheral region. For tube sections where the narrowing extends only to a partial peripheral region, it means that the tube section is lowered more in the direction toward its central axis in one segment of this partial peripheral region than in another segment.
Particularly advantageous is a catalytic converter according to the invention in combination with an expanding mat since mats of this type are considerably cheaper as compared to mineral fiber mats without exfoliated mica. A specific operating temperature must be achieved with such expanding mats, so that the expansion of the exfoliated mica particles is triggered. In the low load range, for example for large-volume diesel engines or for diesel and Otto engines with direct fuel injection, the expansion temperature is frequently not reached. As a result, the monolith is positioned in the catalytic converter housing solely by the initial restoring forces of the expanding mat, which are determined by the gap measure and the original mat thickness. With an expanding mat, the mineral fibers and the exfoliated mica particles are embedded in an organic binder. The organic binder mainly determines the initial restoring force of such a mat. Above approximately 160xc2x0 C., the binder softens and in the process is distributed over the porous structure of the mat. The result is a loss of restoring force or a drop in the radial pressure forces exerted onto the monolith. Added to this is the fact that the binder hardens through partial oxidation as a result of being admitted with heat over a longer period of time at the aforementioned temperature range. This results in an additional and considerable reduction in the axial forces of pressure. This is further reinforced by the fact that vibrations resulting from the vehicle operation cause a steady compression of the positioning mat in radial direction and, as a result of the hardened binder, practically cause a permanent deformation or condensing of the mat. It can go so far as to release the monolith from its anchorage owing to the axial accelerations caused by the engine vibrations and exhaust gas pulsations. With a catalytic converter according to the invention that is produced according to the invention, however, this problem is prevented by selecting the inside cross-sectional surface of the narrowed housing section such that the positioning mat is compressed or pre-stressed to such a degree that a reliable holding of the monolith is ensured, even in the aforementioned low-load ranges or for engines with low heat development. Finally, the reduction in the inside cross-sectional surface can be selected so as to compensate for the production tolerances of the monolith and the tube section, which have an enlarging effect on the gap space, and thus lower the ejection force to below an operationally safe desired minimum value. Thus, the invention offers the further advantage of being able to dispense with an individual calibration of the tube sections. For a calibration of this type, an individual monolith is assigned to each tube section, the cross-sectional surface of the monolith is determined and the tube section is enlarged to achieve the desired gap measure.